U.S. patent application number 16/109246 was filed with the patent office on 2020-02-27 for mitigating low surface quality.
This patent application is currently assigned to Coherent Munich GmbH & Co. KG. The applicant listed for this patent is Coherent Munich GmbH & Co. KG. Invention is credited to Urs EPPELT, Anthony S. Lee, Ludger MULLERS.
Application Number | 20200061750 16/109246 |
Document ID | / |
Family ID | 67587736 |
Filed Date | 2020-02-27 |
United States Patent
Application |
20200061750 |
Kind Code |
A1 |
EPPELT; Urs ; et
al. |
February 27, 2020 |
MITIGATING LOW SURFACE QUALITY
Abstract
Methods and apparatuses are disclosed for laser processing. A
method includes providing a laser beam transparent to a workpiece.
A cover, having a surface quality better than the workpiece's
surface, is provided and spaced apart from the workpiece's surface.
A fluid is provided between and in contact with the cover and the
workpiece's surface. A laser beam is directed through the cover and
fluid to the workpiece. An apparatus includes a cover spaced apart
from a workpiece's surface and including a surface quality better
than the workpiece's surface, a fluid dispenser for introducing
fluid between and in contact with the cover and the workpiece's
surface, and a laser system that directs a laser beam through the
cover and fluid to the workpiece.
Inventors: |
EPPELT; Urs;
(Furstenfeldbruck, DE) ; MULLERS; Ludger;
(Munchen, DE) ; Lee; Anthony S.; (Petaluma,
CA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Coherent Munich GmbH & Co. KG |
Gilching |
|
DE |
|
|
Assignee: |
Coherent Munich GmbH & Co.
KG
Gilching
DE
|
Family ID: |
67587736 |
Appl. No.: |
16/109246 |
Filed: |
August 22, 2018 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
B23K 26/146 20151001;
B23K 2103/54 20180801; B23K 26/0006 20130101; C03B 33/0222
20130101; B23K 26/3576 20180801; B23K 26/122 20130101; B23K 26/53
20151001; B23K 26/009 20130101 |
International
Class: |
B23K 26/352 20060101
B23K026/352; B23K 26/00 20060101 B23K026/00; C03B 33/02 20060101
C03B033/02 |
Claims
1. A method of laser processing a workpiece having a workpiece
surface, the method comprising: providing a laser beam, wherein the
laser beam has a wavelength at which the workpiece is transparent;
providing a cover spaced apart from the workpiece surface, wherein
the cover has a surface proximal to the workpiece surface and a
surface distal to the workpiece surface, and wherein the distal
surface of the cover has a surface quality better than a surface
quality of the workpiece surface; providing a fluid between and in
contact with the proximal surface of the cover and the workpiece
surface; and directing the laser beam through the cover, through
the fluid, and through the workpiece surface.
2. The method of claim 1, wherein directing the laser beam
comprises focusing the laser beam and forming a defect in the
workpiece.
3. The method of claim 1, wherein the fluid has a refractive index
matching the refractive index of the workpiece.
4. The method of claim 1, wherein the fluid has a refractive index
that is between a refractive index of the workpiece and a
refractive index of the cover.
5. The method of claim 1, wherein the distal surface of the cover
has a lower surface roughness than a surface roughness of the
workpiece surface.
6. The method of claim 5, wherein the distal surface of the cover
has a surface roughness of less than 20 Angstroms.
7. The method of claim 1, wherein the distal surface of the cover
has a lower surface irregularity than a surface irregularity of the
workpiece surface.
8. The method of claim 7, wherein the laser beam has a wavelength
.lamda. and wherein the distal surface of the cover has a surface
irregularity of less than .lamda./4.
9. The method of claim 1, wherein the proximal surface of the cover
is parallel to the workpiece surface.
10. The method of claim 1, wherein an optical axis of the laser
beam is incident normally onto the workpiece surface.
11. The method of claim 1, wherein an optical axis of the laser
beam is incident onto the workpiece surface at a non-normal
angle.
12. The method of claim 1, wherein the proximal surface and distal
surface of the cover are parallel.
13. The method of claim 1, wherein the proximal surface and distal
surface of the cover are mutually inclined.
14. The method of claim 13, with the proximal surface of the cover
being parallel to the workpiece surface, and wherein directing the
laser beam comprises focusing the laser beam to form a defect in
the workpiece and oblique to the workpiece surface.
15. The method of claim 1, wherein the distal surface of the cover
has a convex shape.
16. The method of claim 1, wherein the cover is made of glass.
17. The method of claim 1, with the fluid being an oil.
18. The method of claim 1, with the cover being a foil and the
fluid being an adhesive having a refractive index matching a
refractive index of the workpiece.
19. The method of claim 1, wherein directing the laser beam
comprises focusing the laser beam at a location inside the
workpiece.
20. The method of claim 1, wherein directing the laser beam further
comprises directing the laser beam through a second surface of the
workpiece and focusing the laser beam at a location outside the
workpiece.
21. The method of claim 1, further comprising repeating the
directing the laser beam while moving the workpiece relative to the
laser beam and adding fluid between the proximal surface of the
cover and workpiece surface.
22. The method of claim 1, further comprising removing the fluid
after the directing the laser beam.
23. A laser processing apparatus comprising: a cover, spaced apart
from a workpiece surface, having a surface proximal to the
workpiece surface and a surface distal to the workpiece surface,
wherein the distal surface of the cover has a surface quality
better than a surface quality of the workpiece surface; a fluid
dispenser configured to introduce fluid between and in contact with
the proximal surface of the cover and the workpiece surface; and a
laser system configured to direct a laser beam through the cover,
through the fluid, and through the workpiece surface, wherein the
laser beam has a wavelength at which the workpiece is
transparent.
24. The apparatus of claim 23, wherein the laser beam is further
configured to focus the laser beam and form a defect in the
workpiece.
25. The apparatus of claim 23, wherein the fluid has a refractive
index matching a refractive index of the workpiece.
26. The apparatus of claim 23, wherein the fluid has a refractive
index between a refractive index of the workpiece and a refractive
index of the cover.
27. The apparatus of claim 23, wherein the distal surface of the
cover has a lower surface roughness than a surface roughness of the
workpiece surface.
28. The apparatus of claim 27, wherein the distal surface of the
cover has a surface roughness of less than 20 Angstroms.
29. The apparatus of claim 23, wherein the distal surface of the
cover has a lower surface irregularity than a surface irregularity
of the workpiece surface.
30. The apparatus of claim 29, wherein the laser beam has a
wavelength .lamda. and wherein the distal surface has a surface
irregularity of less than .lamda./4.
31. The apparatus of claim 23, wherein the proximal surface of the
cover is parallel to the workpiece surface.
32. The apparatus of claim 23, wherein an optical axis of the laser
beam is incident normally onto the workpiece surface.
33. The apparatus of claim 23, wherein an optical axis of the laser
beam is incident onto the workpiece surface at a non-normal
angle.
34. The apparatus of claim 23, wherein the proximal surface and
distal surface of the cover are parallel.
35. The apparatus of claim 23, wherein the proximal surface and
distal surface of the cover are mutually inclined.
36. The apparatus of claim 35, with the proximal surface of the
cover being parallel to the workpiece surface, and wherein the
laser system is configured to form a defect in the workpiece and
oblique to the workpiece's surface
37. The apparatus of claim 23, wherein the distal surface of the
cover has a convex shape.
38. The apparatus of claim 23, wherein the cover is made of
glass.
39. The apparatus of claim 23, with the fluid being an oil.
40. The apparatus of claim 23, with the cover being a foil and the
fluid being an adhesive having a refractive index matching a
refractive index of the workpiece.
41. The apparatus of claim 23, wherein the laser system is
configured to focus the laser beam at a location inside the
workpiece.
42. The apparatus of claim 23, wherein the laser system is
configured to direct the laser beam through a second surface of the
workpiece and focus the laser beam at a location outside the
workpiece.
43. The apparatus of claim 23, further comprising a translation
stage, and wherein the laser system is configured to repeatedly
direct the laser beam through the fluid and through the workpiece
surface while the translation stage translates the workpiece
relative to the laser beam and fluid is added between the proximal
surface of the cover and workpiece surface.
44. The apparatus of claim 23, further comprising a fluid removal
system configured to remove the fluid from the workpiece after the
laser beam is directed through the fluid and through the workpiece
surface.
Description
TECHNICAL FIELD
[0001] The present disclosure relates in general to processing
materials using laser-radiation. The disclosure relates in
particular to laser processing workpieces with low surface
quality.
BACKGROUND
[0002] Laser material-processing is increasingly used for cutting,
drilling, marking, and scribing a wide range of materials,
including brittle materials such as glass, ceramics, silicon, and
sapphire. Traditional mechanical processing produces unwanted
defects, such as micro-cracks that may propagate when the processed
brittle material is stressed, thereby degrading and weakening the
processed brittle material. Laser-processing of brittle materials
using focused beams of laser-radiation produces precise cuts and
holes, having high-quality edges and walls, while minimizing the
formation of such unwanted defects. Progress in scientific research
and manufacturing is leading to laser-processing of an increasing
range of brittle materials, while demanding increased
laser-processing speed and precision.
[0003] Transparent brittle materials interact with focused beams of
pulsed laser-radiation through non-linear absorption of the
laser-radiation. The pulsed laser-radiation may include a train of
individual pulses, or rapid bursts of pulses. Each individual pulse
or burst of pulses creates a defect in a workpiece of transparent
brittle material at the focus of the beam. An article is cut from
the workpiece by translating the focused beam to create a row of
defects along a cutting line in the workpiece.
[0004] Often the row of defects just weakens the material along the
cutting line. To fully separate the article from the rest of the
workpiece requires an additional step of applying stress across the
cutting line. Applying mechanical stress or thermal stress usually
causes separation along the cutting line. Precise and controlled
separation has been demonstrated using a laser-beam having a
wavelength that is absorbed by the material and relatively high
average power. The absorbed laser-power creates a thermal gradient
across the cutting line, which causes cracks to propagate between
the discrete defects produced by the pulsed laser-radiation,
thereby forming a continuous break along the cutting line.
[0005] By way of example, a highly focused beam of ultra-short
laser-pulses creates a self-guiding "filament" in a glass
workpiece. To create a filament, a focused beam of pulsed
laser-radiation having a sufficiently high intensity in a material
becomes further focused due to non-linear components of the
refractive index. Positive feedback between non-linear
self-focusing and the high-intensity laser beam creates a plasma. A
lower refractive index within the plasma and/or scattering of the
focused beam by the plasma causes defocusing. A balance between the
focusing and the defocusing sustains the plasma within a filament,
which propagates through the glass workpiece and has a diameter
much smaller than a diffraction-limited diameter of the focused
beam of pulsed laser-radiation.
[0006] Propagation of such a filament creates a long slender defect
through the workpiece in the form of a void, micro-cracking, or
other material modifications. A row of defects is created by
translating the focused ultra-short pulsed laser-beam along the
cutting line. A carbon dioxide (CO.sub.2) laser having wavelengths
of around 10 micrometers (.mu.m) can then be used to separate
glass, by translating the CO.sub.2 laser-beam along the cutting
line. Such a laser-cutting process is described in U.S. Pat. Nos.
9,102,007 and 9,296,066, each thereof commonly owned with the
present application, and the complete disclosure of each is
incorporated herein by reference for all purposes.
BRIEF SUMMARY
[0007] Laser material-processing requires a precisely-positioned
and tightly-controlled focus of the laser beam. Relatively small
variances in material properties (such as normal material
inhomogeneities) can cause a loss of focus control. Non-planar
material surfaces can defocus a laser beam due to refraction,
reducing the intensity of the laser beam at the intended focus. It
is possible for the beam intensity to be reduced below a threshold
for the intended material processing.
[0008] Practitioners of skill in the art use "surface quality" as a
measure of these variations. "Surface quality" has two
contributions: small-scale surface structure, referred to as
"surface roughness" or "surface finish;" and large scale structure,
referred to as "surface irregularity" or "surface flatness."
[0009] Small-scale surface structure, having high spatial
frequency, causes optical losses. Usually these are scattering
losses, which reduce the optical power reaching the processing
location after a laser beam is transmitted through the surface.
This "surface roughness" or "surface finish" is quantified by
R.sub.a (average deviation from a mean plane of the surface) or
R.sub.RMS (average maximum peak-valley deviation over a prescribed
surface area)
[0010] Large-scale surface structure, having low spatial frequency,
causes wavefront distortion. By way of example, this wavefront
distortion prevents a focused laser beam transmitted through the
surface from forming a well-defined focus. This "surface
irregularity" or "surface flatness" may be quantified by counting
interference fringes of a monochromatic test beam when the surface
contacts another known flat surface. Therefore, deviation from an
ideal flat surface is measured in multiples of the wavelength
.lamda. of the test beam.
[0011] Existing solutions do not account for laser processing a
workpiece when a laser beam is directed through a surface of the
workpiece having low surface quality, leading to loss of focus
control.
[0012] In one aspect, a method is disclosed for laser processing a
workpiece having a workpiece surface. The method includes providing
a laser beam having a wavelength at which the workpiece is
transparent. A cover is provided and spaced apart from the
workpiece surface. The cover has a surface proximal to the
workpiece surface and a surface distal to the workpiece surface,
where the distal surface has a surface quality better than a
surface quality of the workpiece surface. A fluid is provided
between and in contact with the proximal surface and the workpiece
surface. A laser beam is directed through the cover, through the
fluid, and through the workpiece surface.
[0013] In one aspect, a laser processing apparatus includes a
cover, a fluid dispenser, and a laser system. The cover can be
spaced apart from a workpiece's surface and includes a surface
proximal to the workpiece surface and a surface distal to the
workpiece surface. The distal surface has a surface quality better
than a surface quality of the workpiece surface. The fluid
dispenser is configured to introduce fluid between and in contact
with the proximal surface and the workpiece surface. The laser
system is configured to direct a laser beam through the cover,
through the fluid, and through the workpiece surface, and the laser
beam has a wavelength at which the workpiece is transparent.
BRIEF DESCRIPTION OF THE DRAWINGS
[0014] FIG. 1A is a photograph of a laser processed workpiece
having low surface quality. FIG. 1B is a photograph of a laser
processed workpiece having low surface quality in accordance with
an embodiment of the present invention.
[0015] FIG. 2A is a schematic of a laser apparatus processing a
workpiece having a high surface quality. FIG. 2B is an enlarged
schematic of a portion of FIG. 2A. FIG. 2C is a schematic of a
laser apparatus processing a workpiece having a low surface
quality.
[0016] FIG. 3 is a schematic of a laser apparatus processing a
workpiece having a low surface quality in accordance with an
embodiment.
[0017] FIG. 4 is a schematic of a laser apparatus processing a
workpiece having a low surface quality in accordance with an
embodiment.
[0018] FIG. 5 is a schematic of a laser apparatus processing a
workpiece having a low surface quality in accordance with an
embodiment.
[0019] FIG. 6 is a schematic of a laser apparatus processing a
workpiece having a low surface quality in accordance with an
embodiment.
[0020] FIG. 7 is a laser processing method in accordance with an
embodiment.
DETAILED DESCRIPTION
[0021] Methods and apparatuses described herein position a cover
and fluid between a laser system and a workpiece surface, with the
cover including a distal surface having a surface quality better
than a surface quality of the workpiece surface. Embodiments
described herein reduce ray scattering due to refraction through a
surface having low surface quality, thereby increasing control of
the position and size of a laser beam's focus inside or outside a
workpiece. They enable formation of a focus with an intended
intensity distribution when a converging beam of laser radiation
must traverse a surface having low surface quality. Methods and
apparatuses described herein may be advantageous where, for
example, a workpiece inherently includes low surface quality (e.g.,
drawn glass or unpolished glass), other processing steps reduce
surface quality of a workpiece's surface (e.g., microfabrication of
semi-conductor devices), and in any laser process that requires a
tightly controlled laser focus.
[0022] Herein, "focus" refers to tight foci and to elongated foci,
which are both used in laser material-processing. A tight focus can
be formed by focusing optics having relatively short focal lengths
that cause minimal aberration of the laser beam. An elongated focus
can be formed by focusing optics that deliberately cause aberration
of the laser beam. By way of example, an elongated focus can be
created by filling the clear aperture of a focusing lens having
spherical aberration. Alternatively, an aspheric focusing lens may
be configured to form an elongated focus having a uniform intensity
distribution along the optical axis, as described in U.S. patent
application Ser. No. 15/352,385 (U.S. Patent Publication No.
2018/0133837), which is commonly owned with the present
application. An elongated focus has advantages in laser-cutting,
because the focused laser-radiation is distributed to favor
creation of long defects that extend through the full thickness of
the workpiece.
[0023] Turning now to the drawings, where like features are
designated by like reference numerals, FIG. 1A is a photograph 100
of a side, cross-sectional view of a workpiece having a surface 102
with low surface quality. Photograph 100 depicts a cut edge after
laser processing and separation. The workpiece in FIG. 1A was drawn
glass manufactured by Corning, Inc. under the brand name
Gorilla.RTM. Glass. The glass in FIG. 1A measured approximately 1.1
mm thick (from top to bottom, as depicted in photograph 100) and
the depicted section measured approximately 1.5 mm wide. A
SmartCleave.RTM. Optic (sold by Coherent, Inc. of Santa Clara,
Calif.) having a nominal focal length of 15 mm was used to focus a
pulsed laser beam having bursts of pulses. The SmartCleave.RTM.
Optic created an elongated focus. Each burst had four pulses and
the pulsed laser beam had a wavelength of 1.064 .mu.m. The total
burst energy was about 500 .mu.J. Individual pulses within a burst
were separated by 25 ns, corresponding to a pulse repetition rate
of 40 MHz. It was intended that each burst would form an extended
defect that weakens the material. The defects were intended to be
spaced at intervals of 5 .mu.m.
[0024] The cross-section in photograph 100 is in a plane traversed
by the optical axis of the pulsed laser beam. Dark regions 104
represent areas where the laser beam formed defects that weakened
the material. Clear regions 106 represent incomplete or interrupted
defects, i.e., where the material was not weakened. As can be seen
in photograph 100, unprocessed regions where there are no defects
106 are frequent and irregular. Although the workpiece in FIG. 1A
was sufficiently weakened to be separated, the workpiece was prone
to chipping because of the frequent regions of unprocessed
material, which may cause off-axis fracturing of the workpiece
during separation. Further, the inconsistencies in the laser
processing made the separation unpredictable, yielding unacceptable
variations from cut to cut.
[0025] The inventors posited that the low surface quality of
surface 102 of the workpiece adversely affected the laser system's
(not shown) ability to produce a precisely-positioned and
tightly-controlled focus, thereby forming the incomplete or
interrupted defects. FIG. 1B is a photograph 150 of a side,
cross-sectional view of another workpiece of the same material as
the workpiece in FIG. 1A, but processed with herein disclosed
embodiments of laser processing apparatuses and methods that reduce
the effect of a workpiece's low surface quality. Other than a
region 154 in close proximity to surface 152, the workpiece
presents primarily processed regions 156, demonstrating that an
appropriately controlled focus facilitated the laser processing.
This workpiece is more likely to separate cleanly than the
workpiece in photograph 100 in FIG. 1A, yielding a more predictable
cut with less chips.
[0026] The improved results depicted in FIG. 1B may be particularly
suitable to workpieces with, for example, surfaces having low
surface quality (e.g., drawn glass, workpieces roughened in other
processing steps, etc.) and for applications that require highly
precise positioning and control of the focus. Although FIGS. 1A and
1B depict processing in a plane, the workpiece may be processed
along any straight or curved cutting line by directing the optical
axis of the laser beam accordingly.
[0027] FIG. 2A is a schematic of laser processing apparatus 200. In
FIG. 2A, a workpiece 212A with surface 213A is exposed to a focused
beam of pulsed laser-radiation 214 from laser processing apparatus
200. Focusing of pulsed laser-radiation 214 is indicated by
converging rays 216A and 216B, representing the boundary rays of
the focused beam of laser-radiation. Beam of pulsed laser-radiation
214 is generated by a source of pulsed laser-radiation 218 and has
a wavelength at which workpiece 212A is transparent. Beam of pulsed
laser-radiation 214 is a beam of repeated individual laser-pulses
(here, only three shown) or repeated bursts of lasers pulses. Each
pulse or each burst of pulses creates a defect 220A in the
workpiece. An array 222 of defects 220A is created by translating
workpiece 212A laterally with respect to beam of pulsed
laser-radiation 214, as indicated by the arrow. The focused beam
traces a cutting line 224, which follows the outline of an item to
be cut from the workpiece.
[0028] Apparatus 200 further includes an optional beam-steering
optic 226, an optional beam-conditioning optic 228, and a focusing
lens 230. FIG. 2A depicts beam-steering optic 226 as a plane mirror
arranged to intercept beam of pulsed laser-radiation 214 from
laser-source 218 and direct it towards workpiece 212A.
Beam-conditioning optic 228 is depicted as an afocal beam-expander
arranged to intercept directed beam of pulsed laser-radiation 214
and expand it to mostly fill focusing lens 230. Focusing lens 230
is depicted as a plano-convex lens that is arranged to intercept
expanded beam of pulsed laser-radiation 214 and bring it to focus
in workpiece 212A. Other beam-steering optics and beam-conditioning
optics could also be used.
[0029] Focusing lens 230 could be a single-element lens as depicted
or a multi-element lens assembly. Workpiece 212A is depicted being
translated with respect to a stationary focused beam of pulsed
laser-radiation 214. Alternatively, galvanometer-actuated mirrors
could be included in beam-conditioning optic 228 and a flat-field
objective lens used for focusing lens 230, thereby enabling focused
beam of pulsed laser-radiation 214 to be translated with respect to
a stationary workpiece 212A.
[0030] FIG. 2B is an enlarged schematic of laser beam 214
interacting with workpiece 212A. In the embodiment of FIG. 2A and
FIG. 2B, surface 213A of workpiece 212A has high surface quality.
Beam 214 is represented by rays incident on surface 213A and has an
optical axis 234 normal to surface 213A. As the rays of beam 214
pass from gas above workpiece 212A into workpiece 212A, a
difference in refractive index between the gas and workpiece 212A
causes the rays to refract. Because surface 213A of workpiece 212A
has high surface quality, each ray refracts by a predictable angle,
determined by the ray's incident angle to the surface. Because
surface 213A has high quality, the intensity distribution of focus
232A is precise and tightly controlled, resulting in the formation
of defects 220A. It should be noted that the gas above workpiece
212A could be ambient air or could be an assist gas or assist
gas-mixture selected to improve the laser processing.
[0031] FIG. 2C is an enlarged schematic of laser beam 214
interacting with another workpiece 212B. In FIG. 2C, workpiece 212B
has surface 213B of low surface quality. Workpiece 212B is exposed
to beam of pulsed laser-radiation 214 from apparatus 200. Surface
213B scatters the rays of beam 214 because of unpredictable
refraction by the low quality surface. Ray scattering produces an
uncontrolled focus. For example, in the case of tight focusing, ray
scattering produces a focus having a poorly defined beam waist
location and beam waist diameter. In the case of elongated
focusing, ray scattering produces a focus having an anomalous
intensity distribution along and about optical axis 234. In
particular, the scattering reduces the beam intensity at or around
intended focus 232B. If the surface quality of surface 213B is too
low, the scattering may thereby prevent laser processing.
[0032] In laser filament processing, for example, the scattering
may reduce the intensity of the laser beam at the intended focus
232B below a threshold for non-linear self-focusing, preventing
formation of filaments. When filaments do form, the anomalous
intensity distribution along optical axis 234 may lead to the
creation of incomplete and irregular defects 220B. Under such
conditions, laser filament processing would produce frequent and
irregular unprocessed regions, like those shown in photograph 100
of FIG. 1A.
[0033] Embodiments disclosed herein can produce the superior laser
processing of FIG. 1B for a workpiece with low surface quality
(such as surface 213B in FIG. 2C). FIG. 3 is a cross-sectional view
of laser processing apparatus 300 processing a workpiece in
accordance with such an embodiment. Apparatus 300 includes cover
302, spaced apart from surface 213B of workpiece 212B, and a fluid
dispenser (not shown) configured to introduce fluid 306 between and
in contact with proximal surface 304B of cover 302 and surface 213B
of workpiece 212B. (As used herein, "distal" and "proximal" cover
surfaces are located with respect to the workpiece, unless
otherwise stated.) A laser system (not shown) directs a laser beam
214 through cover 302, through fluid 306, and into workpiece
surface 213B. In some embodiments, the laser processing apparatus
includes the laser system described above with respect to FIGS.
2A-2C. In some embodiments, cover 302 is contained within a laser
processing apparatus that further includes a laser system and a
fluid dispenser configured to introduce fluid 306 between and in
contact with cover 302 and a workpiece.
[0034] Distal surface 304A of cover 302 has a surface quality
better than surface 213B of workpiece 212B. As used herein, a first
surface has a better surface quality than a second surface when the
first surface's surface roughness is lower than the second
surface's surface roughness and/or the first surface's surface
irregularity is lower than the second surface's surface
irregularity. In some embodiments, distal surface 304A has an
optical quality equivalent to a surface having roughness of less
than 20 .ANG. (Angstroms) and/or irregularity of less than
.lamda./4, where .lamda. is the wavelength of the laser beam. In
some embodiments, distal surface 304A has an optical quality
equivalent to roughness of less than 5 .ANG. and/or irregularity of
less than .lamda./20. As used herein, "surface quality" refers to
those areas of a workpiece where the laser beam is incident on a
surface of the workpiece.
[0035] As the rays of beam 214 pass through distal surface 304A, a
difference between the refractive indices of the gas and cover 302
causes the rays to refract. Because distal surface 304A of cover
302 has a better surface quality than surface 213B of workpiece
212B, the rays will refract more predictably than if the rays
passed through surface 213B depicted in FIG. 2C. This leads to more
controlled and predictable foci, which results in more accurate
laser processing, such as the cuts shown in FIG. 1B.
[0036] Fluid 306 can flow to occupy the troughs of the rough
surface of workpiece 212B, resulting in a cover/fluid/workpiece
arrangement with distal surface 304A serving as the interface of
the gas and the cover/fluid/workpiece arrangement for incident
laser beam 214. Fluid 306 is selected such that the difference
between the refractive indices of fluid 306 and workpiece 212B is
smaller than the difference between the refractive indices of the
gas and workpiece 212B. This selection reduces refraction as the
laser beam passes through surface 213B, thereby reducing
undesirable scattering of the rays. Both cover 302 and fluid 306
are selected to be transparent at the wavelength of laser beam 214.
Ray scattering can be further reduced, as explained below.
[0037] To minimize reflective losses through the
cover/fluid/workpiece arrangement, it is preferable to select a
cover that has a refractive index less than or equal to the
refractive index of the workpiece. The fluid would be preferably
selected to have a refractive index that is between the refractive
index of the workpiece and the refractive index of the cover. To
further minimize reflective losses, one or both of the proximal and
distal surfaces of the cover may have an antireflection
coating.
[0038] In some embodiments, fluid 306 has a refractive index
matching the refractive index of workpiece 212B. As used herein, a
refractive index matches another refractive index when they are
less than 10% different from one another. In some embodiments, a
fluid's refractive index is less than 3% different from a
workpiece's refractive index. Matching the refractive indices of
fluid 306 and workpiece 212B reduces or eliminates refraction at
surface 213B of workpiece 212B. In some embodiments, cover 302 has
a refractive index matching the refractive indices of both fluid
306 and workpiece 212B. After passing through distal surface 304A,
the rays would pass through cover 302, fluid 306, and workpiece
212B without changing direction due to the constant (or near
constant) refractive index.
[0039] In some embodiments, the cover's thickness is chosen so that
the cover is sufficiently resilient to prevent warping or changes
in position. In some embodiments, the cover thickness and fluid
thickness are chosen to minimize the distance between the cover's
distal surface and the workpiece surface. Minimizing the distance
between the cover's distal surface and the workpiece surface
maximizes an effective working distance of the laser system.
Specifically, here, the working distance between focusing lens 230
(depicted in FIG. 2A) and the cover. Minimizing the distance
between the cover's distal surface and the workpiece surface also
minimizes the change in depth-of-focus in the workpiece compared to
focusing into the workpiece alone, as depicted in FIGS. 2A-2C. In
some embodiments, the fluid has a minimum thickness greater that
the peak-to-peak roughness of the workpiece.
[0040] In some embodiments, cover 302 is made of a glass. In some
embodiments, a cover is made of soda lime glass. By way of example,
the cover used to capture FIG. 1B was soda lime glass having a
thickness of about 300 .mu.m. In some embodiments, the cover is
made of fused silica, or any transparent material having the
required surface quality. The cover material could be selected to
meet any other application requirements, for example, a chemically
resistant glass.
[0041] As depicted in FIG. 3, an optical axis 234 of laser beam 214
is incident normally onto surface 213B of workpiece 212B. In some
embodiments, an optical axis of a laser beam is incident on a
surface of a workpiece at a non-normal angle (for example, see FIG.
6 described below).
[0042] As depicted in FIG. 3, distal surface 304A is planar and an
optical axis 234 of laser beam 214 is incident normally onto distal
surface 304A. In some embodiments, distal surface 304A is
non-planar. In some embodiments, optical axis 234 of laser beam 214
is incident onto a distal surface of a cover at a non-normal
angle.
[0043] As depicted in FIG. 3, distal surface 304A and proximal
surface 304B are parallel. As used herein, the term "parallel" is
understood to include deviations from perfectly parallel that do
not affect an application of the laser beam. For example, a
deviation of two surfaces perfectly parallel is within the term
"parallel" if the deviation is not so large as to change the focus
depth when translating the beam from one side of a workpiece to the
other, such that the process would exceed application tolerances.
In some embodiments, distal surface 304A and proximal surface 304B
are not parallel (for example, see FIG. 6 described below).
[0044] As depicted in FIG. 3, proximal surface 304B is parallel to
workpiece surface 213B. In some embodiments, proximal surface 304B
is not parallel to workpiece surface 213B.
[0045] In some embodiments, fluid 306 includes a liquid, a gel, a
malleable polymer, or a conformable solid. In some embodiments,
fluid 306 is an oil. Exemplary oils to match a workpiece made of
Gorilla.RTM. Glass having a refractive index of about 1.51 at 1064
nanometers include IM01-immersion oil/IM02-immersion oil
(refractive index 1.48-1.482), glycerin (refractive index 1.46),
and Olympus immersion oil (refractive index 1.51). In some
embodiments, the cover is a transparent foil (e.g., PVC) and the
fluid is an adhesive having a refractive index matching a
workpiece's refractive index.
[0046] In some embodiments, laser beam 214 has a wavelength at
which workpiece 212B is transparent. As used herein, an object is
"transparent" to a laser beam when all or a portion of the laser
beam's power incident on an object's surface is transmitted to a
location below the object's surface. For example, an object is
transparent to a laser beam when 40% of incident laser power is
transmitted to a location below the object's surface or an object
is transparent to a laser beam when 70% of incident laser power is
transmitted to a location below the object's surface. For example,
a workpiece is transparent when at least 40% of incident laser
power is transmitted to the location of a focus.
[0047] In some embodiments, the laser system is configured to form
a focus 232A at a location inside workpiece 212B. In some
embodiments, the laser system is configured to direct the laser
beam 214 through a second opposite surface of the workpiece and
form a focus outside the workpiece. For example, below the lower
surface (in the orientation depicted in FIG. 3) of workpiece
212B.
[0048] In some embodiments, an apparatus includes a translation
stage configured to move the workpiece relative to the laser beam
and the fluid dispenser is configured to introduce fluid between
the cover and workpiece while the workpiece moves relative to the
laser beam (see FIG. 4 and FIG. 5 below).
[0049] In some embodiments, the apparatus includes a fluid removal
system configured to remove fluid 306 from workpiece 212B after the
laser beam 214 has processed workpiece 212B. In such embodiments, a
volatile index matched fluid may be used for efficient and complete
fluid removal.
[0050] In some embodiments, the laser system is configured to focus
the laser beam to form a filament and thereby create a defect 220A
in workpiece 212B. In some embodiments, laser processing apparatus
300 is used for other laser processes, such as stealth dicing
(e.g., processing of silicon at a wavelength of about 1 .mu.m).
Laser processing apparatus 300 may be advantageous in, for example,
any laser material processing requiring good beam integrity,
particularly high intensity and/or fine control of beam
parameters.
[0051] FIG. 4 is a cross-sectional view of a laser processing
apparatus 400 in accordance with an embodiment. Laser processing
apparatus 400 includes fluid supply line 402, fluid dispenser 404,
cover 302, and a laser system (which includes focusing optic 230).
Fluid dispenser 404 receives fluid 306 from a fluid reservoir (not
shown) and provides the fluid between and in contact with cover 302
and workpiece 212B.
[0052] Laser beam 214 passes through cover 302, through fluid 306,
and into workpiece 212B. In the embodiment depicted in FIG. 4,
laser beam 214 forms a focus 232A located inside workpiece 212B and
forms a defect 220A. Translation of the laser beam with respect to
the workpiece creates an array of defects 222 in workpiece 212B.
Exemplary systems and methods for laser filament processing are
described in described in U.S. Pat. Nos. 9,102,007 and 9,296,066,
each thereof commonly owned with the present application, and
incorporated herein by reference. Such a laser-cutting process
SmartCleave.RTM. is licensed by Coherent, Inc.
[0053] Fluid dispenser 404 incorporates the cover 302, at least one
fluid supply line 402, and at least one fluid reservoir (not
shown). The shape of the bottom surface of the dispenser that
includes the cover could be round, rectangular, or any shape
suitable for the application. In some embodiments, fluid dispenser
404 is either a part of the laser-processing head or is attached to
the head. Fluid is dispensed by a pump, capillary action, and/or
gravity. For a pump embodiment, the pumps (not shown) may include
an adjustable pump speed that is varied in combination with the
translation speed (of the workpiece) to create a desired fluid feed
between the cover and the workpiece. A fluid film may remain on the
workpiece after laser processing. As described above with respect
to FIG. 3, a laser processing apparatus may include a fluid removal
system.
[0054] FIG. 5 is a cross-sectional view of laser processing
apparatus 500 in accordance with an embodiment. Laser processing
apparatus 500 is similar to laser processing apparatus 400, and the
discussion of FIG. 4 applies to FIG. 5, and vice versa. Differences
include a fluid dispenser 502 with two fluid lines 402 inclined to
the plane of the cover 302, and that fluid dispenser 502 is a
separate assembly.
[0055] Laser beam 214 passes through cover 302, through fluid 306,
and into workpiece 212B. In the embodiment depicted in FIG. 5,
laser beam 214 forms a focus 232A located inside workpiece 212B and
forms a defect. Translation of the laser beam with respect to the
workpiece creates an array of defects 222 in workpiece 212B.
[0056] FIG. 6 is a cross-sectional view of laser processing
apparatus 600 processing a workpiece in accordance with an
embodiment. Laser processing apparatus 600 is similar to laser
processing apparatus 300 described above with respect to FIG.
3--that description applies equally to laser processing apparatus
600, and vice versa. The differences between laser processing
apparatus 600 and laser processing apparatus 300 include the shape
of cover 602 and the angle of inclination of workpiece 212B with
respect to laser beam 214.
[0057] As shown in FIG. 6, optical axis 234 of laser beam 214 is
incident on surface 213B of workpiece 212B at a non-normal angle.
In this arrangement, directing the laser beam comprises focusing
the laser beam to produce a laser defect in the workpiece that is
oblique to the workpiece surface. This embodiment can be used for
surfaces with low surface quality (shown) or with high surface
quality. It enables an inclined workpiece or an inclined section of
a workpiece to be laser processed. In the case of laser
filamentation, it would also allow filaments to be formed and
defects to be created that are inclined with respect to the
surfaces of the workpiece.
[0058] Cover 602 includes a distal surface 604A upon which laser
beam 214 is incident normally and proximal surface 604B that is
parallel to surface 213B of workpiece 212B. Distal surface 604A and
proximal surface 604B are thus mutually inclined and cover 602 has
a wedge or prism shape. In other embodiments, the cover may have a
different shape, provided that the distal and proximal surface of
the workpiece are mutually inclined. Again, a fluid 606 is between
and in contact with proximal surface 604B and surface 213B. For
example, a variety of proximal surface-distal surface relative
inclinations (including a parallel arrangement) are available so
that a cover can be moved to accommodate a variety of cuts on a
workpiece.
[0059] FIG. 6 shows relative translation of the workpiece with
respect to the prism-shaped cover and focused laser beam. Some
embodiments translate the prism-shaped cover and the focused laser
beam, while the workpiece is stationary.
[0060] FIG. 7 is a flow diagram of laser processing method 700 in
accordance with an embodiment. Method 700 is a method of laser
processing a workpiece having a workpiece surface and includes:
providing a laser beam 702, where the laser beam has a wavelength
at which the workpiece is transparent; providing a cover 704 spaced
apart from a workpiece surface, wherein the cover has a surface
proximal to the workpiece surface and a surface distal to the
workpiece surface, and wherein the distal surface has a surface
quality better than a surface quality of the workpiece surface;
providing a fluid 706 between and in contact with the proximal
surface and the workpiece surface; and directing the laser beam 708
through the cover, through the fluid, and through the workpiece
surface. As used herein, "directing the laser beam" is understood
to include any movement of the laser beam relative to the
workpiece. For example, "directing the laser beam" includes moving
a laser system while keeping the workpiece stationary, moving the
workpiece while keeping a laser system stationary, or scanning a
laser beam laterally with respect to the workpiece. Optionally,
method 700 may loop from directing the laser beam 708 to providing
a fluid 706, such as, for example, when the workpiece is translated
relative to the laser beam.
[0061] In some embodiments of the method, directing the laser beam
includes focusing the laser beam and forming a defect in the
workpiece.
[0062] In some embodiments, the fluid has a refractive index that
is between a refractive index of a gas above the cover and a
refractive index of the workpiece. In some embodiments, the fluid
has a refractive index that is between a refractive index of the
cover and a refractive index of the workpiece. In some embodiments
of the method, the fluid has a refractive index matching the
refractive index of the workpiece.
[0063] In some embodiments of the method, the distal surface has a
lower surface roughness than a surface roughness of the workpiece
surface. In some embodiments of the method, the distal surface has
a surface roughness of less than 20 .ANG..
[0064] In some embodiments of the method, the distal surface has a
lower surface irregularity than a surface irregularity of the
workpiece surface. In some embodiments of the method, the laser
beam has a wavelength .lamda. and the distal surface has a surface
irregularity of less than .lamda./4.
[0065] In some embodiments of the method, the proximal surface is
parallel to the workpiece surface.
[0066] In some embodiments of the method, an optical axis of the
laser beam is incident normally onto the workpiece surface.
[0067] In some embodiments of the method, wherein an optical axis
of the laser beam is incident onto the workpiece surface at a
non-normal angle.
[0068] In some embodiments of the method, the proximal surface and
distal surface are parallel. In some embodiments of the method, the
proximal surface and distal surface are mutually inclined. In some
embodiments of the method, the proximal surface is parallel to the
workpiece surface, and directing the laser beam comprises focusing
the laser beam to produce a defect in the workpiece and oblique to
the workpiece surface.
[0069] In some embodiments of the method, the distal surface has a
convex shape.
[0070] In some embodiments of the method, the cover is glass. In
some embodiments of the method, the fluid is an oil. In some
embodiments of the method, the cover is a foil and the fluid is an
adhesive having a refractive index matching a refractive index of
the workpiece.
[0071] In some embodiments of the method, directing the laser beam
comprises focusing the laser beam at a location inside the
workpiece. In some embodiments of the method, directing the laser
beam further comprises directing the laser beam through a second
surface of the workpiece and focusing the laser beam at a location
outside the workpiece.
[0072] Some embodiments of the method further comprise repeatedly
directing the laser beam while moving the workpiece relative to the
laser beam and adding fluid between the proximal surface and
workpiece surface.
[0073] Some embodiments of the method further comprise removing the
fluid after the directing the laser beam.
[0074] In some embodiments, focused beam of pulsed laser-radiation
214 converges to a focus that is elongated along optical axis 234,
as discussed above. Referring to FIGS. 2A and 2B, rays close to
optical axis 234 converge closer to or further from focusing lens
230 than boundary rays 216A and 216B, thereby extending the focus
along the optical axis. Workpiece 230 would be located such that
the elongated focus overlaps or at least partially overlaps with
the workpiece. Defects 220A are depicted extending through most of
the thickness of workpiece 213A. For cutting applications, in
particular, it is preferable for the defects to extend through the
full thickness of the workpiece. In general, the length of an
elongated focus defines the length of the defects, provided each
burst of pulses has sufficient energy.
[0075] In some embodiments, the cover surfaces may be non-planar.
Although cover 302 is depicted in FIGS. 3-5 as a sheet and cover
602 is depicted in FIG. 6 as a prism, the cover may have a
plano-convex shape such that each ray is incident normally on the
cover's distal surface 304A. In accordance with the present
invention, a distal surface 304A having a convex shape would have a
higher surface quality than surface 213B of workpiece 212B. This
plano-convex shape may be advantageous when, for example, a
workpiece has a high refractive index, to minimize reflective
losses through the cover/fluid/workpiece arrangement.
[0076] As discussed herein above, in some embodiments, the laser
processing apparatus is configured to direct laser beam 214 through
surface 213B and through a second opposite surface of workpiece
212B. In these embodiments, a second cover may be spaced apart from
the second surface, and having a fluid filling the space
thereinbetweeen. This arrangement allows a focus to be formed
outside a workpiece with opposing surfaces that both have low
surface quality. An external focus is favorable in some
applications. For example, to form a defect that extends to a
surface may require an elongated focus that traverses the
surface.
[0077] Some embodiments include an additional step of exposing a
workpiece to a beam of laser-radiation generated by a source of
laser-radiation different from laser-source 218 of FIG. 2A. The
beam of laser-radiation from a different source may have a
wavelength that is absorbed by the workpiece 212. The workpiece may
be translated laterally with respect to the beam of different
laser-radiation and the beam heats the material weakened by defects
220A, causing it to crack completely and creating a cut-edge.
Exposing a workpiece to a beam from a second source of laser
radiation causing a workpiece to crack is described in more detail
in U.S. application Ser. No. 15/913,457, incorporated by reference
herein in its entirety for all purposes.
[0078] The present invention is described above with reference to
preferred and other embodiments. The invention is not limited,
however, to the embodiments described and depicted herein. Rather,
the invention is limited only by the claims appended hereto. The
use herein of "including," "comprising," "having," "containing,"
"involving," and variations thereof is meant to encompass the items
listed thereafter and equivalents thereof as well as additional
items.
* * * * *